Dielectric (or insulating) fluids used in electrical distribution and power equipment—including transformers, switching gear and electric cables—perform two important functions. These fluids act as an electrical insulating medium, i.e., exhibit dielectric strength, and they transport generated heat away from the equipment, i.e., act as a cooling medium. When used in a transformer, for example, dielectric fluids transport heat from the windings and core of the transformer or connected circuits to cooling surfaces. Apart from possessing dielectric strength and cooling capacity, an ideal dielectric fluid for electrical equipment also exhibits little or no detrimental impact on the environment, is compatible with materials used to construct the equipment, and is relatively nonflammable.
For more than a century, mineral oils derived from crude petroleum were used extensively as insulating and cooling liquids in electrical equipment. Though such oils possess a satisfactory dielectric strength and are compatible with equipment materials, they are not considered nonflammable, and, because they are petroleum-based, they are considered to carry with them an environmental cost. In the middle part of this century, as safety standards became more demanding for many indoor and vault equipment installations, mineral oils were replaced to a large extent by nonflammable liquids such as polychlorinated biphenyl (PCB) fluids. PCBs eventually were recognized for their environmental hazards, and as a result the production and sale of PCBs as well as their use in new equipment was banned.
Because of the disadvantages and shortcomings of PCB-based fluids and because of the increasing sensitivity to the potential adverse environmental impact of mineral oils and available alternatives, there have been and continue to be numerous efforts undertaken to develop relatively inexpensive, environmentally safe, and nonflammable dielectric fluids.
Due to an increasing awareness and sensitivity toward environmental concerns, it has become increasingly desirable to provide a dielectric fluid that: (1) poses minimal environmental hazards; (2) degrades quickly and easily so that spills do not contaminate the soil or the water table for any significant period of time; and (3) does not interfere in any significant way with natural biodegradation processes.
It also is becoming more desirable to replace non-renewable resources with renewable resources, particularly given the undesirability of dependence on petroleum-derived products, and there generally is increased demand by the industrial and retail markets for all-natural products. This is due, at least in part, from the attention paid to the long-term effects of materials and their degradation by-products.